December 14, 2000: In this Hubble telescope picture, a curtain of glowing gas is wrapped around Jupiter's north pole like a lasso. This curtain of light, called an aurora, is produced when high-energy electrons race along the planet's magnetic field and into the upper atmosphere where they excite atmospheric gases, causing them to glow. The aurora resembles the same phenomenon that crowns Earth's polar regions. But this Hubble image, taken in ultraviolet light, also shows the glowing "footprints" of three of Jupiter's largest moons: Io, Ganymede, and Europa. Over the next two months, Jupiter's aurora will be scrutinized by two observatories: the Hubble telescope and the Cassini spacecraft, which will fly by the planet on its voyage to Saturn.
Each footprint is represented by a bright dot. Io's footprint is at far left; Ganymede's is just below and to the right of center; and Europa's is to the right of Ganymede's signature. These emissions, produced by electric currents generated by the moons, flow along Jupiter's magnetic field, bouncing in and out of the upper atmosphere. They are unlike anything seen on Earth.